The present invention relates to a compact superposed modulated signal generator for supplying a compact superposed modulated signal for use in a digital transmission system to transmit digital information, and more particularly to a compact superposed modulated signal generator by which the signal to be transmitted has a narrow occupied bandwidth and low distortion of the modulated waveform even if transmitted by a non-linear amplifier. Such a compact superposed modulated signal generator is useful in satellite communication systems, mobile radio systems, and terrestrial microwave communication links, which require more information to be transmitted within a limited transmission bandwidth and at higher power efficiency.
Conventional digital transmission systems normally deal with binary data signals in the non-return-to-zero (NRZ) form and modulate a single carrier or quadrature carrier, and then transmit the modulated carrier to receivers via transmission channels such as a satellite or microwave communication link. In order to obtain sufficient transmission power at the output stage, the signal is amplified by a high power amplifier (HPA). The HPA is required to operate in its saturation region so that the system may more efficiently utilize its power. Operation in the saturation region is particularly necessary for mobile transmission systems which need much longer transmission time using limited power. Further, in order for more than one user to transmit signals at the same time within a limited bandwidth, a bandwidth efficient signal is required for the modulated signal.
The quadrature phase shift keying (QPSK) method which limits the bandwidth using a raised cosine filter is widely used in most of the digital transmission systems owing to its very narrow occupied bandwidth in a linear channel and simple hardware implementation. However, when a QPSK modulated signal is band-limited by a filter and transmitted by an HPA operating in the saturation region, transmission spectral side lobes at the output of HPA regrow due to the non-linear amplification of the HPA. The regrown side lobes cause serious degradations into adjacent channels, and the probability of error increases. Therefore, for power and bandwidth efficient applications, QPSK method is not a desirable modulation technique.
To achieve power and bandwidth efficient digital transmission systems, the power spectrum of a non-linearly amplified modulated signal should have a narrow main lobe and low regrowth in its side lobes. To decrease probability of error, timing jitter and inter-symbol interference of the received signal should be minimized.
Among the recent studies on power and bandwidth efficient modulation techniques, the inventions of U.S. Pat. No. 4,339,724 titled "Filter" (IJF-OQPSK) by Dr. Kamilo Feher and U.S. Pat. No. 4,644,565 titled "Superposed Quadrature Modulated Baseband Signal Processor" (SQAM) by Dr. J. S. Seo and Dr. K. Feher relate to a modulated signal generator for detecting two consecutive bits of an input NRZ signal and generating a double interval raised cosine pulse waveform according to the input data format. The latter invention is a technology capable of controlling main and side lobes by superposing by raised cosine pulse signal of the former invention with a single interval raised cosine pulse of a different amplitude.
The aforementioned two modulation techniques have lower spectral regrowth due to a non-linearity of HPA than do QPSK, offset-QPSK, and better probability of error performance. However, since their respective main lobe occupied bandwidths are wider than that of a QPSK modulating system band-limited by a conventional raised cosine filter, the probability of error may increase if the distance between the desired and adjacent channels is very narrow.